[ms-cxxabi] bitcast to i8* to deref a data member pointer

This was causing us to miscompile
llvm::SymbolTableListTraits::getListOwner(), which uses data member
pointers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196545 91177308-0d34-0410-b5e6-96231b3b80d8